1. Field of the Invention
The present invention relates to a semiconductor device which is formed by using a semiconductor film having a crystal structure (also referred to as a crystalline semiconductor film) and a method for manufacturing the same, as well as a semiconductor integrated circuit comprising a circuit integrating the semiconductor devices and a method for manufacturing the same. Further, the present invention relates to a semiconductor device comprising a plurality of the semiconductor integrated circuits. And, the present invention especially relates to a semiconductor device included in a thin film transistor whose channel region is formed in a crystalline semiconductor formed on an insulating surface.
2. Description of the Related Art
A technique for forming a semiconductor device such as a thin film transistor and the like using a crystalline semiconductor film formed on an insulation substrate such as glass, has been developed. The thin film transistor formed by using the crystalline semiconductor film is integrated in a semiconductor integrated circuit, and the semiconductor integrated circuit is used in a flat panel display typified by a liquid crystal display device and an EL (electro luminescence) display device.
A current mirror circuit is a basic circuit of a semiconductor integrated circuit comprising a thin film transistor. The principle of the current mirror circuit is to have two thin film transistors with same electric characteristics. Such a circuit structure also can be given as an operational amplifier, a differential amplifier and the like.
As a method for forming a crystalline semiconductor film on an insulating substrate, a technique using laser beams to crystallize an amorphous semiconductor film has been developed. In a semiconductor manufacturing process, such as the technique for crystallizing an amorphous semiconductor film by using laser beams, a gas laser such as an excimer laser, and a solid laser such as a YAG laser are generally used as the laser beam source. An example for crystallizing an amorphous semiconductor film by laser beam irradiation is disclosed in JP-A-62-104117, in which is described a poly-crystallization of the amorphous semiconductor film by high-speed laser beam scan with the scanning speed set to more than 5000 times a diameter of the laser beam spot per second, without totally melting the amorphous semiconductor film. In addition, U.S. Pat. No. 4,330,363 discloses a technique to form a substantially single crystal region by irradiating an extended laser beam on an island-shaped semiconductor film. As another example, a method of irradiating a laser beam formed into a linear-like shape by an optical system, such as a laser processing apparatus, is disclosed in JP-A-8-195357.
Further, JP-A-2001-144027 discloses a technique such that crystalline semiconductor films with large grain size are formed by irradiating laser beams of a second harmonic onto the amorphous semiconductor films using solid laser oscillation apparatus such as Nd: YVO4 laser. A transistor is thereby constituted.
However, when crystallization is made by irradiating the laser beams onto the amorphous semiconductor film, the crystal includes polycrystals which are formed at random and induce the formation of defects such as grain boundaries. Therefore, it becomes difficult to obtain uniform crystallinity and crystal orientations. As a result, current values may vary even when semiconductor devices of the same size are made and the same voltages are applied to the semiconductor devices.
Crystal defects such as grain boundaries result in carrier traps. This may be considered as a causative factor for mobility reduction of electrons or holes. Also, it is impossible to form a semiconductor film with neither distortion nor crystal defects due to factors such as a volume shrinkage of the semiconductor films, a thermal stress occurring between the semiconductor film and a base film, or a lattice mismatching which accompany crystallization. Consequently, the distortion and crystal defects are not only at the origin of inhomogeneous electrical characteristics of the semiconductor device, but are also the cause of inferior electrical characteristics of the semiconductor device.
Especially when crystalline semiconductor films are formed by using laser beams on a non-alkali glass substrate used abundantly and industrially, the focuses of the laser beams vary in response to the influence of the surge of the non-alkali glass substrate itself, causing undesired variations in the crystalline characteristics as a result. Furthermore, in order for a non-alkali glass substrate to avoid contamination by an alkaline metal, it is necessary to prepare protection films such as insulating films, as base films. Yet, it is almost impossible to form thereon crystalline semiconductor films free of grain boundaries and crystal defects.
The semiconductor integrated circuit and the like have semiconductor films formed on cheap glass substrates to constitute transistors, therefore, it is almost impossible to arrange transistors so as to avoid randomly located grain boundaries. That is, the grain boundaries or crystal defects hinder a strict control of the crystallinity of the channel forming regions of transistors. This is a causative factor of variation in the electrical characteristics of the semiconductor device, which make it difficult to form a circuit which requires high conformity (for example, a current mirror circuit).